1. Field of the Invention
This invention relates to a liquid thermosetting filling composition or filler grade composition which is useful as an ink for permanently filling such holes as via holes and through holes in printed circuit boards of a multilayer board or a double-sided board, a sealing compound for IC packages, and the like. This invention further relates to a method for permanently filling holes in printed circuit boards by the use of the composition mentioned above.
2. Description of the Prior Art
In recent years, in the surface mounting of electronic parts on the printed circuit boards, the electronic parts used thereon have been shifting from discrete parts to IC and LSI parts and the IC packages have been shifting from the dual inline package (DIP) type to the flat pack type. In terms of the mode of mounting, the parts have been also shifting on a large scale from the aggregate of function blocks to those of hybrid IC's and further to those having a large number of pins separated at a fine pitch. In consequence of these changes, the printed circuit boards have been advancing toward decreasing the width of pattern lines and contracting mounting area.
Further, with a view to coping with the trend of portable devices toward decreasing size and increasing function, the desirability of producing printed circuit boards in further decreased weight and size has been finding growing recognition. As a result, the printed circuit boards are advancing in the direction of favoring the build-up process which comprises superposing insulating layers one each on the upper and the lower surface of a core member, forming necessary circuits thereon, further repeating the superposition of insulating layers and the formation of circuits, and so on and the mounted parts are advancing in the direction of favoring such an area array type as, for example, the ball grid array (BGA) and the land grid array (LGA).
In the circumstances, the desirability of developing a liquid filling ink to be used in permanently filling via holes in boards such as, for example, inner via holes (IVH) in build-up grade core members, surface via holes (SVH) in built-up insulating layers, and via holes in boards for BGA and LGA and consequently producing hardened fillers excellent in physical properties such as polishability and other characteristics has been finding universal recognition.
As the ink for permanently filling such holes in printed circuit boards, the epoxy resin compositions of the thermosetting type and the UV/heat-curable type have been heretofore introduced. The thermosetting type effects the reaction of the relevant epoxy resin by virtue of heat and the UV/heat-curable type precures the relevant photosensitive compound through the radical polymerization reaction of the double bond thereof during the course of precure and subsequently completes the thermal curing of the epoxy resin during the course of heating.
The epoxy resin compositions have been finding utility in a wide variety of fields covering electrical insulating materials, FRP (fiber-reinforced plastics) and other similar composite materials, coating materials, and adhesive agents because their hardened products excel in mechanical, electrical, and chemical properties, and exhibit good adhesiveness as well. The same remarks hold good for the epoxy resin composition to be used as the ink for permanently filling holes in printed circuit boards. It has been customary for this ink to use a primary or secondary aromatic amine or acid anhydride as a curing agent and a tertiary amine or imidazole as a catalyst. When an aromatic amine is used, however, the resin composition shrinks largely during the course of thermal curing and the hardened filler eventually formed of the resin composition in a through hole gives birth to a gap between itself and the wall of the through hole or gives rise to voids (empty cavities) in the hardened filler filling the hole. When the epoxy resin composition contains a solvent, since the solvent vaporizes while the resin composition is being thermally cured, the hardened filler formed of the resin composition in the through hole has the problem of caving in or cissing. Still another epoxy resin curing system suffers the curing reaction thereof to terminate so instantaneously on account of a chain reaction as to permit no easy control. The resultant hardened filler has so high hardness that it may not be easily polished or cut to a flat surface.
The UV/heat-curable type epoxy resin composition is capable of being precured by being exposed to an ultraviolet light. Since the radical polymerization of the double bond of the photosensitive compound included in the composition such as, for example, acrylate proceeds more quickly in the surface part than in the inner part of the composition, the degree with which the composition is photocured varies in the surface part and the inner part thereof and the shrinkage which the composition sustains during the course of thermal curing is large. Further, the resultant hardened composition possesses hygroscopicity and suffers from the drawback of failing to acquire electrical insulation properties and PCT (pressure cooker treatment) resistance sufficiently.
As respects the curing system which combines an epoxy resin with a phenolic resin, though having no bearing on an ink intended for permanently filling holes in printed circuit boards, published Japanese Patent Application, KOKAI (Early Publication) No. 8-157,561 discloses a semiconductor sealing epoxy resin composition characterized by containing a solid epoxy resin of a specific structure including at least one hydrocarbon group in a benzene ring in combination with an imidazole compound of a specific structure. In this published specification is cited a typical composition using a solid epoxy resin and a solid phenolic resin. The sealing resin which is obtained from this composition exhibits satisfactory special qualities and physical properties after it has been hardened. The composition nevertheless is deficient in the ability to be coated as by screen printing or roll coating because the epoxy resin and the phenolic resin which are raw materials are both in a powdery state. This semiconductor sealing epoxy resin composition, therefore, has not yet found actual adoption as a filling material for via holes in printed circuit boards due to its poor workability and productivity.